Session Issues with Certain ISPs

Hi, I have a fairly simple set of php scripts which use session variables to pass information from one page to the next in an online booking system for an event. In most cases these work fine, but users with a couple of ISPs are consistently unable to use the system, as the session variables seem to get lost along the way. The troublesome ISPs are AOL and BTOpenworld. Does anyone have any experiences that could shed light on this for me? Thanks, Mark